Erasable writing compositions are well known in the art. They can be dispensed through ball point writing instruments as well as other writing devices. The compositions are designed to facilitate removal of script before affixed to a substrate, like paper. For a writing medium to be erasable, the writing composition must temporarily prevent penetration of the substrate by colorant. If penetration is not inhibited, the substrate can be damaged before the writing composition is erased. This erasable feature has necessitated undesirable rheological properties for prior art compositions.
Rheological properties of commercial erasable ink compositions inhibit delivery of a uniform layer of ink to substrates. Simply, commercially available erasable ink compositions used in ball point pens have the disadvantage of not producing a uniform script. Those compositions are hard to start. Ink intensity varies with writing speed; script is not sharp. Typically, this script is characterized by filaments of ink which detract from line sharpness. Such defects apparently result from undesirable rheological properties of the commercially available ink compositions. The rheology widely deviates from Newtonian behavior.
Known commercial erasable ink compositions have undesirable rheological properties. U.S. Pat. No. 4,097,290 to Muller et al. relates to an erasable ink composition which contains natural rubber or rubber which essentially duplicates the chemical structure of natural rubber. It also contains a volatile low-boiling point organic solvent as well as a high-boiling point organic solvent. The composition operates by rapid evaporation of the low-boiling solvent when the composition is applied to substrates. The viscosity of the mixture increases and penetration of the remaining natural rubber and colorant is minimized because the remaining residue thickens. Then, the high-boiling solvent evaporates and the pigmented rubber adheres to and penetrates into the substrate. Then, the ink is no longer erasable.
Another known commercialized erasable ink composition also possesses the aforementioned disadvantages. U.S. Pat. No. 3,875,105 to Daugherty et al. teaches an erasable ink composition containing a low-boiling solvent, a polyvinyl methyl ether and a colorant. Specific concentrations are taught for these components. When the composition is applied to a substrate, such as paper, the solvent quickly evaporates to effect an increase in viscosity. The polyvinyl methyl ether is only soluble in the solvent. When the solvent evaporates, the ether precipitates to capture colorant. Consequently, the colorant remains on the paper and does not penetrate it.
Both of these commercial compositions effect radical changes in rheological properties with respect to conventional inks. These commercial compositions rely upon this effect to acheive temporary erasability of the ink composition.
The present invention seeks to overcome the disadvantages of such compositions. Prior art compositions rely on radical changes in the rheological properties of the compositions which necessitate wide deviation from Newtonian behavior. Rather, the present invention is directed to an erasable ink composition which can approach Newtonian behavior, that is, water-like behavior. Simply, the composition of the present invention can flow on application of a small force and for which its rate of flow is approximately directly proportional to the applied force.